Vitamins and minerals: nitrous oxide and methionine synthase: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Vitamins and minerals: nitrous oxide and methionine synthase

Sample item

A 24-year-old who inhales nitrous oxide cartridges daily has paraesthesia, impaired vibration sense and a broad-based gait. Serum cobalamin is low-normal and plasma homocysteine is raised. Which step does the gas directly disable?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Oxidation of cobalt in cobalamin, inactivating methionine synthase

Nitrous oxide irreversibly oxidises the cobalt of methylcobalamin, which inactivates methionine synthase. Homocysteine accumulates, methyl-folate is trapped and subacute combined degeneration can follow. Serum cobalamin may look normal because the vitamin is present but non-functional.

Option by option

  • A. Conversion of homocysteine to cystathionine

    Cystathionine beta-synthase is a pyridoxal phosphate enzyme and is not the target of the gas.

  • B. Oxidation of cobalt in cobalamin, inactivating methionine synthase (correct)

    Correct. The oxidised cobalamin cannot serve methionine synthase, so homocysteine rises.

  • C. Absorption of cobalamin bound to intrinsic factor

    Ileal absorption is not affected by an inhaled gas.

  • D. Reduction of dihydrofolate by its reductase

    Dihydrofolate reductase is the target of methotrexate and trimethoprim, not of nitrous oxide.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Oxidation of cobalt in cobalamin, inactivating methionine synthase. Nitrous oxide irreversibly oxidises the cobalt of methylcobalamin, which inactivates methionine synthase.

Why is "Conversion of homocysteine to cystathionine" not the answer?

Cystathionine beta-synthase is a pyridoxal phosphate enzyme and is not the target of the gas.

Why is "Absorption of cobalamin bound to intrinsic factor" not the answer?

Ileal absorption is not affected by an inhaled gas.

Why is "Reduction of dihydrofolate by its reductase" not the answer?

Dihydrofolate reductase is the target of methotrexate and trimethoprim, not of nitrous oxide.

What concept does this question test?

It tests Vitamins and minerals: nitrous oxide and methionine synthase in Biochemistry. Homocysteine accumulates, methyl-folate is trapped and subacute combined degeneration can follow. Serum cobalamin may look normal because the vitamin is present but non-functional.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Vitamins and minerals: nitrous oxide and methionine synthase?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.